1. Field of the Invention
The present invention relates to a polytetrafluoroethylene (PTFE) real twist yarn and a method of producing the same.
2. Description of Related Art
Since PTFE resins have an extremely high melting viscosity and they do not dissolve in most solvents, PTFE fibers cannot be produced by using generally-adopted methods such as extrusion spinning of molten resins and resin solutions. For this reason, a variety of special production methods have been conventionally adopted. U.S. Pat. No. 2,772,444 proposes a method of producing PTFE fibers by emulsion spinning of a mixed solution of an aqueous dispersion solution of PTFE fine particles and viscose, followed by sintering of the PTFE at high temperatures to remove the viscose by thermal decomposition. However, the cost of producing PTFE fibers by using this method is high, whereas the strength of the fibers obtained is low, and therefore the strength of products obtained by processing the fibers as a raw material is also low.
U.S. Pat. Nos. 3,953,566 and 4,187,390 propose a method of obtaining PTFE fibers by slitting a PTFE film or sheet into a minute width, followed by stretching of the obtained tape. However, with regard to this method, it is difficult to maintain the width of the tape obtained by slitting the film or sheet uniformly along the length direction. Also an end portion of the tape tends to be fibrillated. For these reasons, some fibers get ruptured during a step of stretching the tape to a high degree.
Further, JP 2004-244787 A and JP 2006-124899 A propose a method of obtaining high-strength PTFE fibers by slitting a PTFE stretched film or sheet into a minute width, followed by stretching of the obtained tape. However, similarly to the above method, an end portion of the tape tends to be fibrillated, and as a result, some fibers get ruptured during a step of stretching the tape to a high degree.
Further, JP H07-500386 A discloses a yarn obtained by helically rolling a PTFE film or sheet and causing the film or sheet to adhere to itself. The yarn has a spiral seam on its surface along the length direction. However, due to having a spiral seam on its surface, this yarn gets snagged on other materials easily and also is vulnerable to friction. Further, U.S. Pat. No. 5,765,576 discloses a PTFE filament whose surface area is increased by towing in order to make the filament friction resistant. However, the problem of this PTFE filament is that it has a flat cross-section.
When the above-described conventional PTFE real twist yarns are made into sewing yarns, they get snagged on a sewing machine needle easily as they have spiral joints on the side or have a flat cross-section. Further, when the conventional PTFE real twist yarns are made into dental flosses, since consumers may prefer a dental floss with a circular cross-section, they cannot meet such a preference.